This application claims priority from Korean Patent Application No. 2002-32558 filed on Jun. 11, 2002 in the Korean Intellectual Property Office, the disclosures of which are incorporated herein in their entirety by reference.
1. Field Of The Invention
The present invention relates to a cantilever of an atomic force microscope for measuring a surface state of a sample, and more particularly, to a Fabry-Perot resonator using an optical fiber having an end of a concave mirror shape and a system for measuring a displacement of a cantilever tip and keeping the displacement of the cantilever tip constant by use of the Fabry-Perot resonator.
2. Description of the Related Art
An atomic force microscope (AFM) disclosed in U.S. Pat. No. 6,032,518 is a high resolution surface measuring instrument. The AFM scans the surface of a sample while maintaining an interval between a tip fixed at an end of a cantilever which is a head for checking the sample surface and the sample at a few nanometers (10xe2x88x929 m), and measures movement in a vertical (Z-axis) direction of the tip, i.e., the height of the sample surface, by using the deflection of the cantilever depending on a variation in the height of the sample surface. Then the AFM controls the height of a fixed part of the cantilever by using the height of the sample surface as a feedback signal, thereby keeping the deflection of the cantilever constant.
A method for measuring a displacement of a tip using the above AFM can be classified into two in principle.
FIG. 1 is a diagram illustrated to describe one method for measuring a displacement of a tip in an AFM. A laser beam irradiated from a laser light source (not shown) and then reflected at a reflective surface 102 of a cantilever 101 is detected by a position sensing detector 103, thereby measuring a variation in reflected positions of the laser beam. Then an angular deflection of the cantilever 101 is measured based on the variation in positions of the laser beam.
In order to precisely measure the angular deflection of the cantilever 101, the cantilever 101 should be far from the position sensing detector 103. Due to this restricted condition, it is difficult to manufacture a small-sized head or raise measurement sensitivity.
FIG. 2 is a diagram illustrated to describe another method for measuring a displacement of a tip in an AFM. A laser beam coming from a laser light source (not shown) is irradiated at a reflective surface 202 of a cantilever 201 through an optical fiber 203. A distance between an end 204 of the optical fiber 203 and the reflective surface 202 of the cantilever 201 is measured by using interferences between a laser beam reflected at the end 204 of the optical fiber 203 and that reflected at the reflective surface 202 of the cantilever 201. If this measuring method is used, it is possible to manufacture the small-sized head of the AFM and have high resolution in principle.
However, an actually measured value does not reach theoretical resolution because the end of the optical fiber is parallel to the reflective surface of the cantilever and thus the intensity of the laser beam returning to the optical fiber by being reflected at the reflective surface of the cantilever is weak.
In a typical single-mode optical fiber, since a core through which light passes is only a few micrometers (10xe2x88x926 m) in diameter, a probability that light irradiated at the reflective surface of the cantilever returns to the optical fiber is low and the strength of a signal is also weak. That is, the signal returning to the optical fiber is vulnerable to electric and mechanical noises and resolution is deteriorated.
It is therefore an object of the present invention to provide a Fabry-Perot resonator for satisfying its resonance condition irrespective of the deflection or warp of a cantilever by using an optical fiber having an end of a concave mirror shape.
It is another object of the present invention to provide a system for precisely measuring a distance between an optical fiber and a cantilever and keeping the distance constant by raising the strength and sensitivity of a signal reflected at the cantilever by using the Fabry-Perot resonator.
It is still another object of the present invention to provide a system for minimizing the size of a head of an AFM, by separating a light source and a position sensing detector from a cantilever and constructing the head of the AFM only with the cantilever and an optical fiber.
According to one aspect of the present invention, a Fabry-Perot resonator using an optical fiber having an end of a concave mirror shape includes a cantilever having an upper side of a reflective surface and having a lower side with a tip for touching a sample, for moving in a Z-axis direction along with a head body by interlocking a piezoelectric element fixed at a column of an atomic force microscope, an optical fiber having one end connected to a light source for irradiating a laser beam, and the other end of a concave mirror shape apart from the reflective surface of the cantilever by a predetermined distance, and an optical detector for detecting a signal reflected at the end of the concave mirror shape of the optical fiber through a directional coupler positioned at a predetermined location of the optical fiber and a signal incident through the end of the concave mirror shape of the optical fiber after being reflected at the reflective surface of the cantilever.
Preferably, the optical fiber is away from the cantilever by 1-10 micrometers.
According to another aspect of the present invention, a system for measuring a displacement of a tip of a cantilever using a Fabry-Perot resonator in an atomic force microscope, includes a cantilever having an upper side of a reflective surface and having a lower side with a tip for touching a sample, for moving in a Z-axis direction along with a head body by interlocking a piezoelectric element fixed at a column of an atomic force microscope, an optical fiber having one end connected to a light source for irradiating a laser beam, and the other end of a concave mirror shape apart from the reflective surface of the cantilever by a predetermined distance, an optical detector for detecting a signal reflected at the end of the concave mirror shape of the optical fiber through a directional coupler positioned at a predetermined location of the optical fiber and a signal incident through the end of the concave mirror shape of the optical fiber after being reflected at the reflective surface of the cantilever, and a signal processor for calculating an error signal proportional to a displacement of the cantilever from the signals detected from the optical detector, and thereby obtaining a variation in a distance between the end of the concave mirror shape of the optical fiber and the reflective surface of the cantilever.
Preferably, the optical fiber is away from the cantilever by 1-10 micrometers.
Preferably, the signal processor calculates the error signal between a predetermined reference value and strength of the signal received from the optical detector, and obtains the variation in the distance between the end of the concave mirror shape of the optical fiber and the reflective surface of the cantilever by using the error signal.
Preferably, the signal processor calculates a differential value of the signal received from the optical detector, and obtains the variation in the distance between the end of the concave mirror shape of the optical fiber and the reflective surface of the cantilever by using the differential value.
According to still another aspect of the present invention, a system for calibrating a displacement of a tip of a cantilever using a Fabry-Perot resonator in an atomic force microscope, includes a cantilever having an upper side of a reflective surface and having a lower side with a tip for touching a sample, for moving in a Z-axis direction along with a head body by interlocking a piezoelectric element fixed at a column of an atomic force microscope, an optical fiber having one end connected to a light source for irradiating a laser beam, and the other end of a concave mirror shape apart from the reflective surface of the cantilever by a predetermined distance, an optical detector for detecting a signal reflected at the end of the concave mirror shape of the optical fiber through a directional coupler positioned at a predetermined location of the optical fiber and a signal incident through the end of the concave mirror shape of the optical fiber after being reflected at the reflective surface of the cantilever, a signal processor for calculating an error signal proportional to a displacement of the cantilever from the signals detected from the optical detector, obtaining a variation in a distance between the end of the concave mirror shape of the optical fiber and the reflective surface of the cantilever, and generating a feedback signal to calibrate and keep the distance therebetween constant, and a servo circuit part for moving the cantilever and the optical fiber in a Z-axis direction by actuating the piezoelectric element by the feedback signal generated from the signal processor, so that the distance between a sample and the tip of the cantilever is kept constant.
Preferably, the optical fiber is away from the cantilever by 1-10 micrometers.
Preferably, the signal processor calculates the error signal between a predetermined reference value and strength of the signal received from the optical detector, and obtains the variation in the distance between the end of the concave mirror shape of the optical fiber and the reflective surface of the cantilever by using the error signal.
Preferably, the signal processor calculates a differential value of the signal received from the optical detector, and obtains the variation in the distance between the end of the concave mirror shape of the optical fiber and the reflective surface of the cantilever by using the differential value.